The present disclosure relates to a solar cell system, an electronic device, and a structure, and a solar cell system suitable for use with a dye-sensitized solar cell, for example, and an electronic device and a structure having the solar cell system.
Solar cells as photoelectric conversion elements for converting solar light into electric energy use solar light as an energy source. Solar cells therefore have a very small effect on a global environment, and are expected to spread further. Crystalline silicon based solar cells using single crystal silicon or polycrystalline silicon and amorphous silicon based solar cells have been mainly used as solar cells in the past.
On the other hand, a dye-sensitized solar cell proposed by Gratzel et al. in 1991 has been drawing attention because the dye-sensitized solar cell can provide high photoelectric conversion efficiency, and can be manufactured at low cost without a need for large-scale equipment at a time of the manufacturing unlike silicon based solar cells in the past (see Nature, 353, pp. 737-740, 1991, for example).
The dye-sensitized solar cell generally has a structure including porous electrodes made of titanium oxide (TiO2) or the like to which a photosensitizing dye is bonded and an electrolytic layer of an electrolytic solution filled between these electrodes. An electrolytic solution obtained by dissolving an electrolyte including redox species such as iodine (I2), iodide ions (I−), and the like in a solvent is often used.
A solar cell has a current-voltage output characteristic (I-V output characteristic) such that the value of a current that can be extracted is determined by the value of voltage of a load connected to the solar cell. The power generation output P of the solar cell is expressed by a product of the power generation voltage V and the power generation current I of the solar cell. Thus, when the solar cell is in an open-circuit state, for example, the power generation current I does not flow, that is, I=0, so that the power generation output P is P=0. When the solar cell is in a short-circuit state, on the other hand, the value of the power generation current I becomes very large, but the power generation voltage V is V=0, so that, again, the power generation output P is P=0. That is, for efficient power generation of the solar cell, the load connected to the solar cell should not be a zero load as in the open-circuit state nor an overload as in the short-circuit state, and it is very important that the load be an appropriate load.
FIG. 21 is a diagram showing an example of the I-V output characteristic and the power-voltage output characteristic (P-V output characteristic) of the solar cell under a constant-light source.
As shown in FIG. 21, an axis of abscissas indicates the power generation voltage V occurring in the solar cell, and an axis of ordinates indicates the power generation current I of the solar cell and generated power output P. When attention is directed to the P-V output characteristic, an operating point that maximizes the output power from the solar cell is the vertex of the P-V output characteristic, and is generally referred to as a maximum power point (MPP). That is, it can be said that highest power generation efficiency is achieved when the solar cell is generating power at the MPP at all times.
However, the P-V output characteristic of the solar cell varies greatly depending on the intensity of solar radiation, temperature, load conditions, and the like, and the MPP correspondingly varies greatly. Thus, maximum power point tracking (MPPT) control is necessary to make the solar cell generate power with high efficiency at all times. Specifically, the MPPT control finds the MPP as a combination of the power generation voltage V and the power generation current I maximizing the power generation output P, continues applying an appropriate load to the solar cell so as to maintain that state at all times, and thereby makes the solar cell operate at a maximum efficiency at all times. In other words, the MPPT control is to convert solar energy into electric energy without a waste, and is in fact one of controls indispensable for driving the solar cell.
Electronic circuits used as MPPT control circuits are roughly classified into following two kinds of systems. One is a circuit that sets the power generation voltage as a control variable and performs feedback control so that the value of the power generation voltage is a set value. The other is a circuit that sets the power generation current as a control variable and performs feedback so that the value of the power generation current is a set value. These control methods are referred to as potential regulating control (Potentiostatic Control) and current regulating control (Galvanostatic Control), respectively, in a field of electrochemistry.
When an MPPT control circuit is actually designed, the former potential regulating control is used overwhelmingly. A reason for this is explained from the dependence of the power generation voltage and the power generation current on illuminance. The solar cell is generally desired to operate stably in a range of illuminance extending by a few orders of magnitude from an illuminance of about 0.5 W/m2 within an office to an illuminance of about 1000 W/m2 under direct sunlight in midsummer. By reason of the characteristics of the solar cell, the power generation current Imax at the MPP is substantially proportional to illuminance. On the other hand, the power generation voltage Vmax at the MPP is substantially proportional to the logarithm of illuminance. That is, the power generation current varies sharply by a few orders of magnitude with changes in illuminance, whereas the power generation voltage is logarithmically compressed and thus varies in a reduced range. For this reason, the power generation voltage is easy to handle as a control variable, and a control circuit becomes simpler when controlling the power generation voltage.
A control method referred to as a hill climbing method has been known in the past among methods of the MPPT control. The hill climbing method changes the set value of the power generation voltage V or the power generation current I at certain time intervals by increasing or decreasing the set value of the power generation voltage V or the power generation current I, checks whether the power generation output P has been raised or dropped by the change, and determines whether to raise or lower the power generation voltage for a next time according to a result of the checking. An MPPT control method based on the hill climbing method is often used for power generation control on solar cells, and a large number of reports have heretofore been made on the technology (for example, Japanese Patent Laid-Open No. Hei 7-234733, Japanese Patent Laid-Open No. Hei 8-76865, Japanese Patent Laid-Open No. 2002-48704, and Japanese Patent Laid-Open No. 2004-280220).